RELATION OF RHYOLITES TO SANDSTONES. 211 



lavas and plutonic rocks, we are justified in believing that they are 

 connected in origin. This connection is at present best made out in 

 the case of the granites and felsites or altered rhyolites. For, first, 

 there are the experiments of Sir James Hall and others, proving that 

 when granite is melted and cooled slowly it forms a substance like 

 felsite, and when it cools more rapidly the product is a glass like 

 obsidian. And, secondly, there are the descriptions by Professor 

 Judd of the tertiary felsitic rocks of the inner Hebrides, which can 

 be shown to have originated from extinct volcanoes of which the 

 central cores are granites ; and the same fact is demonstrated in 

 the primary period, by the occurrence of felsites in Scotland under 

 circumstances which show that they have been derived from granite 

 masses. Therefore the step is sufficiently probable which would 

 lead us to derive felsites and all the imperfectly crystallised or glassy 

 forms of granitic rocks from the metamorphism or melting up of 

 certain clays, and their extrusion or ejection on the earth's surface ; 

 but some may have been derived from sandstones. 



Igneous Rocks related to Sandstones. There are no igneous 

 rocks which correspond entirely in chemical composition with the 

 few sandstones which have been examined. Those analysed by Mr. 

 John Arthur Phillips all contain a smaller percentage of alumina 

 than is usual in rhyolites ; but rhyolites vary sufficiently among them- 

 selves in this respect, as may be seen from the analyses collected by 

 Justus Roth, 1 to justify a comparison. The tertiary rhyolite, described 

 by Mr. Hardman, from Tardree, near Antrim, has, however, but 5-10 

 per cent, of alumina to 76*96 per cent, of silica. The analyses of 

 American sandstones correspond with rhyolites very closely ; the only 

 difference being a slight variation in the proportions of potash, and 



soda. "We therefore suggest that rhyolites are often sandstones which 

 have been liquefied and erupted. The way in which the quartz 



1 " Beitriige zur Petrographie der plutonischen Gesteine." 4to. 1873. 



2 In the Rhyolite of Otting, the proportions of the alkalies are reversed, being 

 soda 1-55, potash 5-25. 



